The main advantage of burners in which the fuel is mixed with the hot combustion gases prior to the admixture of combustion air is the fact that the fuel is preheated and diluted with combustion gases and to a certain part decomposed before it comes in contact with the combustion air. This improves combustion considerably.
While such burners work satisfactory at constant output, difficulties are encountered if a wide range or operation is desired. Owing to the fact that the volume of the recirculated combustion gases changes with the velocity or volume of the combustion air no sufficient volume of combustion gases is recirculated at low loads (where also the volume of combustion air is small) although a greater amount of recirculated combustion gases would be desirable.